I will be sailing in Lake Michigan, no lack of wind or room there.
I've been sailing Lake Michigan for the better part of 12 years now. I'd like to know where you are sailing that you have strong and steady enough of wind to use regularly. Also are you going to be sailing in the middle of the lake which is about 30-40 miles out.
Sorry I shouldn't be so snarky. The reality is if you're weekend sailing a wind vane will work for maybe about 30% of the time if your lucky. About half the time when you go out on the weekend in the summer there won't be enough wind to really go anywhere. If the wind is blowing it will usually be from the wrong direction so you'd have to beat to windward, which on a 25ft boat is going to SUCK in Lake Michigan. The waves are short and steep, 3ft waves for any length of time if you're trying to go anywhere are going to be about the limit you'd want to bash into. So when the time comes and you do have wind, and there is enough to move the boat, half the time figure you are running downwind. Now guess what the windvane doesn't work AGAIN.
Trust me you'll be motoring a lot more than you think you will. Which is ok because it's still a day spent on the water. For the price difference between the Raymarine St1000 and this used 20-30 years old windvane is about $100 bucks, not including at least the $100 you'll spend on blocks and line to hook it up. The windvane will work about 10-20% of the time (if it works at all, may need parts that are no longer available). The tiller pilot will work about 90% of the time, as long as it has power. The other 10% the wind will be too high or the waves too much and the weather helm will keep it from being able to hold a course.
In short unless you plan to leave the great lakes and do passages in the trades, where you have constant wind for days on end, get a tiller pilot. Most cruisers that have a windvane will have an autopilot. Why not start with and autopilot and see if you need a windvane. If you're worried about battery capacity get a small solar panel, or maybe your outboard has an alternator already.
You'll get tons of use out of the autopilot. You can interface it with small handheld GPS and have it run way-points for you. You can tap the +10 and -10 button at the same time and have it tack for you while you work the jib sheets. The autopilot will warn you if it's not able to hold a course. All these things will make single handing or sailing with an inexperienced partner so much easier. The windvane will do none of these things.
However if you're set on your windvane go for it. I hope it works out for you and I hope to see you out there sailing.
PS. We sail out of Muskegon MI and live in Grand Rapids. If you have any questions feel free to drop me a line. If you're ever planning on stopping in Muskegon drop me a line, we'll get some Bloody Mary's with Lobster claws in them at Dockers.